There has been conventionally used a technique for embedding, as additional information, information which prohibits transfer of an audio signal to another equipment or which limits recording of the audio signal in order to realize protection of the contents of an audio work. The additional information of this type is embedded into an audio signal as a watermark, which may be a digital watermark or an analog watermark.
As a technique for embedding a digital watermark into a digital audio signal, there is employed a technique which uses the least significant bit (LSB) of a 16-bit PCM audio signal for watermark data. Also, there is employed a technique for embedding additional information into a digital audio signal as a watermark by operating the modified discrete cosine transform (MDCT) coefficient of a compression-coded digital audio signal or the coefficient of a subband.
Since a digital watermark can be read and written by superimposing watermark data directly on a digital audio signal, signal processing is facilitated. However, the digital watermark will be broken when the digital audio signal is demodulated to an analog audio signal. The digital watermark might also be broken when the digital audio signal is converted to a different data format. Therefore, the digital watermark cannot limit repeated recording of the analog audio signal, that is, copying of the analog audio signal, and cannot sufficiently protect the interest of the copyright holder of the audio work.
An analog watermark is embedded into a digital audio signal in such a manner that it is detected in the form of an analog signal. Even after conversion of the file format is carried out, the watermark can be read again by demodulating the digital audio signal to an analog audio signal.
Meanwhile, a technique for distributing an audio work such as a music tune to the user through a communication network is proposed. This distribution technique is exemplified by the electronic music distribution (EMD) for transmitting and recording a digital audio signal in a compressed data format. An analog watermark which is embedded in the compressed digital audio signal distributed by the EMD cannot be read out or written unless the compressed digital audio signal is demodulated to a PCM signal or an analog signal. Therefore, in order to record the audio signal distributed by the EMD on which the analog watermark is superimposed, the user needs to demodulate the audio signal to a PCM signal. As the compressed digital audio signal is demodulated to a PCM signal or the like, the data size is increased and recording to a recording medium cannot be carried out efficiently. Also, in order to rewrite the analog watermark, the audio signal distribution side needs to demodulate audio signal once compressed to a PCM signal and therefore cannot rewrite the analog watermark easily.
As methods for embedding an analog watermark into an audio signal, a spread spectrum system and a phase shift keying (PSK) system are proposed. The spread spectrum system and the PSK system are adapted for embedding additional information to an audio signal by utilizing a masking effect with respect to the auditory sense in reproducing an audio signal. However, since these systems cannot provide a sufficient masking effect, it is difficult to embed the additional information into the audio signal without deteriorating the quality of the reproduced sound.